<!DOCTYPE html>
<html>
<head>
<style>
.content-box {
  width: 80%;
  max-width: 800px;
  margin: 0 auto;
  padding: 20px;
  border: 1px solid #ccc;
  border-radius: 5px;
  background-color: #f9f9f9;
}
</style>
</head>
<body>
  <div class="content-box">
    <h2>内容标题</h2>
    <p>这是内容框里的文本内容。</p>
  </div>
</body>
</html>